In order to increase the yield of crops in greenhouses, supplemental assimilation light is employed next to the natural sunlight. In particular high-intensity discharge lamps, HID lamps, such as SON-T, with an electrical power of 600-1000 W, are employed. As a result, the light level in a greenhouse increases, leading to a higher yield of crop produced per square-meter greenhouse area when compared to greenhouses working only with sunlight. However, plants like tomatoes or cucumbers grow to a height of several meters. In such a case, the upper leaves, being closest to the light source, absorb most of the incident light. The shadow zone formed in this way leads to less efficient use of the lower leaves to photosynthesis and consequently less production. To further increase the yield of crop per square meter greenhouse area, additional assimilation light sources illuminate plants in the shadow zone. This lighting concept is known as intercanopy lighting.
Efficient intercanopy lighting is typically realized by illuminating the plants in a vertical zone that starts in the lowest part of the plant where leafs develop and that stops approximately 0.5 m below the top of the plant. Light reaching the parts of the plant outside of this vertical zone is either lost towards the floor or ceiling or reaches parts of the plant that are already sufficiently illuminated by luminaries positioned above the plant.
Typically, the intercanopy lighting is provided by arranging HID lamps between rows of plants, such as tomatoes or cucumbers, with a predetermined distance from the plant row of typically between 40 cm and 1.0 m with a distance to neighboring lamps of typically 1.9 m. Theses geometric conditions in combination with the lamebrain light distribution of these lamps result in strongly varying horizontal and vertical irradiance on the plant surface. Further, by arranging the lamps between the rows of plants, the intercanopy lighting blocks the working area that has to be accessible to the workers of the greenhouse. Retractable HID lamp systems are known, in which the lamps are temporarily retracted to clear the working area for access by the workers. Subsequently the lamps have to be repositioned to provide the intercanopy lighting.